OBJECTIVE: To study the reproducibility of a food frequency questionnaire used to assess past dietary habits. DESIGN: Repeated retrospective assessment of dietary habits of a population sample. SETTING: Uppsala-Orebro Health Care Region of Sweden. SUBJECTS: One-hundred and four subjects participating as control subjects in a case-control study on thyroid cancer. INTERVENTIONS: Between spring 1993 and spring 1994 a case-control study was conducted in the study area, in order to investigate risk factors for thyroid cancer-diet among others. The study subjects reported their consumption of selected foods with reference 1-5 y before, as well as dietary changes that occurred from adolescence to adult age. One year later 197 control persons were invited to provide a second report of the same food items. Of the invited subjects 104 agreed to participate. RESULTS: The median Spearman rank correlation coefficient between the first and second assessment was 0.58 for food consumption and 0. 56 for nutrients intake. Correlation between the two assessments varied greatly between food items, ranging from -0.03 to 0.83. The correlation was positively related to the frequency and negatively related to the skewness of consumption. The recall of dietary changes from adolescence had a median correlation coefficient of 0. 38 (range 0.19-0.69). There were hints of a higher reproducibility of dietary reports among men and among highly educated subjects (more than 11 y of education), but these differences were statistically significant only for recall of adolescent diet. Reproducibility of nutrients intake was also significantly higher among men than among women, as well as among subjects older than 45 y. CONCLUSIONS: The reproducibility of dietary reports was satisfactory. Among factors affecting reproducibility, frequency and homogeneity of consumption in the source population are presumably the most important. Age, sex and education modify the inter-subject reproducibility of past diet. Based on reproducibility of a semi-quantitative index, the recall of adolescent diet probably has a low validity. European Journal of Clinical Nutrition (2000) 54, 658-664.